This application is filed concurrently with related patent application Ser. No. 07/614,654, now abandoned.
The present invention relates to a boron nitride/boron carbide composite and a method for making said composite material. Use of the present invention in the ceramic art increases the durability of ceramic articles by reducing friction, wear, chipping and fracture of the articles. The present invention can be used in the manufacture of all boron based, or boron containing ceramic materials; and the invention can especially be employed in ceramic components such as bearings or internal moving parts of a ceramic engine. Generally speaking, the present invention is particularly suitable for improving the physical and chemical properties of ceramic work pieces designed to be exposed to excessive wear, erosion, temperatures and fatigue and for work pieces which would benefit from exhibiting a lower coefficient of friction at their surfaces.
Studies have been conducted which indicate that reduction of friction on the surface of materials enhances the materials' durability. Moreover, application to ceramic, for example, of materials having great strength would also increase the life of resulting article. These facts are well known to one skilled in the composite art.
Boron carbide, one of the hardest and most durable materials known to man, has a hardness of approximately 45% that of the hardness of a diamond. In addition, boron carbide has a low specific density. Therefore, it has a high strength to weight ratio. These properties make boron carbide an attractive material for application to articles requiring wear resistance and a high strength to weight ratio, such as cutting tools.
Ion implantation of ceramics has been recently studied and summarized by Bull and Page in Journal of Material Science, Volume 23, p. 4217 (1988). Ion-implantation is a process whereby all near-surface and surface characteristics of materials may be altered. It is used to change the surface chemistry of ceramics in order to influence the mechanical properties thereof. Ion implantation of ceramics is apparently a complex phenomenon. This phenomenon has tribological significance--tribology is the study of friction and wear between interacting parts and involves means of reducing the friction. Although it would appear that ion implantation would be an ideal technique for modifying the wear resistance and durability of materials, such as ceramics, studies conducted indicate that the effects of ion implantation, in general, are unpredictable. Consequently, the effects of ion implantation must be experimentally determined for each material in which it is sought. In addition, the various methods of ion implantation may effect the resulting properties of the final product. Hence, based on the technological knowledge available, all that one skilled in the art of ion implantation can conclude is that both the method of ion implantation and the materials in which ion implantation is sought play a major role in the properties resulting from said treatment. The effects of these considerations, however, are complex and unpredictable. The unpredictable nature of this art area makes residual damage structure modeling difficult.
As an aside, Bull and Page do make mention of ionimplantation combined with some post-implantation annealing. The specific ion-implantation and annealing processes employed within the present invention, however, are nowhere disclosed or hinted at therein.
Studies conducted by Wei et al., Materials Science and Engineering, Volume 90, p. 307 (1987), have inferred that ion beam modification treatments of silicon carbide (SiC) type ceramics and other ceramics reduce the friction and the wear of said ceramics. This sought after property is attained through the resulting formation of a thin layer of lubricious oxides.
The use of laser treatments in the ceramics art has been shown to significantly improve the fracture strength of said materials. A study reported by Singh et al. in Journal of Material Res., Volume 3, p. 1119 (1988), indicated such property improvements. Singh et al, however, makes no mention of the combination of laser treatments with nitrogen ion-implantation to improve durability of ceramics.
Additional studies in tribology were conducted by Nastasi et al. and DeKoven et al. Both groups conducted wear studies of nitrogen implantation of boron carbide; and both research groups came up with conflicting results of the effects of their implantation on the frictional coefficient of the materials used. DeKoven et al., in Surface Coatings Technology, Volume 36, p. 207 (1988), reported a significant increase in the frictional coefficient after nitrogen implantation of boron carbide; whereas, Nastasi et al., in Journal of Material Res., Volume 3, p. 1127 (1988), found a marked reduction in the frictional coefficient after nitrogen implantation into hot-pressed boron carbide (B.sub.4 C). This reduction in frictional coefficient was attributed to the possibility of the formation of nitride-like "bonds." Nastasi et al. further postulated in their report that nitrogen implantation resulted in a thermodynamically more stable chemistry. Nowhere, however, do either Nastasi et al. or DeKoven et al. even remotely suggest the combination of nitrogen ion-implantation with laser or rapid thermal annealing into ceramics. Nor do they quantify the formation of bonding.
The present invention overcomes the unpredictable effects of creating a more durable, ceramic material having reduced friction properties. The present invention enables one skilled in the art of ceramics to produce a consistently durable, wear resistant ceramic product. The present invention involves the novel combination of nitrogen ion implantation with laser or rapid thermal annealing treatments of ceramics to form the product claimed herein. X-Ray Photoelectron Spectroscopy results indicate that there is minimal, insignificant boron - nitride bonding unless both nitrogen ion-implantation and laser or rapid thermal annealing have been performed.